1. Field of the Invention
The present invention relates generally to the field of vibration testing of objects such as satellites, instrumentation or any other object whose reliability in operation may be evaluated using high intensity vibration testing. Specifically, the present invention relates to the use of direct field acoustic systems to perform vibration testing and to control means to allow direct field acoustic systems to closely approximate the properties of a reverberant field.
2. Background of the Invention
Many complex items such as satellites or other instruments are subjected to high levels of vibration, mechanical stress and other extreme conditions during operation or as part of the process of being placed into service. Failures may be difficult or impossible to repair after the object is placed in service and may be extremely costly. As a result there have been many techniques developed to evaluate the reliability of these objects during design, manufacture or certification to avoid in-service failures. Mechanical vibration testing is the most common form of test wherein the unit under test (UUT) is mounted to a vibratory platform driven by one or more types of mechanical transducers, such as hydraulic or electro-dynamic. A vibration control system (VCS) is typically used to provide closed loop control of the test to achieve certain pre-specified test characteristics. Most mechanical testing is “Single-Axis” wherein a single group of commonly driven transducers is used to excite the UUT. However, a more complex control system involving multiple inputs and multiple outputs (MIMO) has also been used for the purpose of delivering specific test conditions to specific parts of the UUT. Such mechanical testing is limited to relatively low frequencies and is unsuitable for evaluating the impact of high intensity acoustic vibrational fields on the UUT.
Historically, the preferred method of evaluating the impact of high intensity acoustic fields has been through the use of a special reverberant high-energy acoustic chamber. These fixed installations use transducers driven by expanding gas to create very high acoustic levels. The reverberant nature of the chamber ensures a uniform but highly uncorrelated acoustic field which is considered necessary for many such tests. While these chambers are capable of very high acoustic levels, in excess of 150 db SPL, across a very broad acoustic bandwidth the acoustic field cannot be accurately controlled at higher frequencies, typically above 2 kHz, due to the nature of the transducers employed, and at low frequencies, typically below 50 Hz, where the low modal density of the chamber causes large spatial variations in the acoustic field. Since these are large fixed installations, very few of which exist, the UUT must typically be transported to the chamber. In the case of satellites, rockets and other such valuable objects transportation is costly, time consuming and risky. Additionally, the lack of control at high and low frequencies may result in over-testing and consequent damage to the UUT.
Accordingly, there has been interest in creating a system for acoustic testing which would offer accurate control of the test conditions across the entire spectrum of interest from approximately 20 Hz to 10 kHz and which could be temporarily assembled and configured on site thereby avoiding the risks of transporting the UUT. The first attempts to implement a so-called “Direct Field Acoustic Test” (DFAT) were carried out in the late 1990's using commercially available sound equipment to surround the UUT with a high intensity acoustic field. The usefulness of these tests was somewhat limited in that the acoustic field generated by the system exhibited substantial spatial variability and a high level of coherence meaning that it did not accurately simulate the desired reverberant field. In addition, the limitations of then available commercial sound equipment made it difficult to reliably achieve high intensity acoustic fields at the desired levels. In spite of these limitations commercial testing using DFAT systems began as early as 1998.
Thereafter, development work on DFAT systems continued. U.S. Pat. No. 6,484,580, issued November 2002 and assigned to Ball Aerospace & Technologies, Corp., incorporated by reference herein in its entirety, discloses the same system and method already in use along with a rudimentary control system based on ⅓rd octave bands. Although no data on coherence is provided, there is no reason to believe that this system was any more successful in simulating a reverberant acoustic field than essentially identical systems previously used.
However, in October of 2003 a paper presented by Larkin and Smallwood at the Aero Space Test Seminar made important advances. Larkin and Smallwood recognized the importance of using fixed band-width narrow-band control for acoustic testing where the band-width of the control bands is a fixed frequency interval, for example a constant 3.125 Hz band-width, rather than a constant portion of an octave, for example, 1/nth octave. They demonstrated that fixed band-width narrow-band control could reveal important acoustic response aberrations normally obscured by averaging over the commonly used ⅓rd octave control bands. They disclosed how acoustic control inputs derived from microphones could be converted to narrow-band power spectral densities (PSD) which could in-turn be used as control inputs to a standard single output random VCS normally used for mechanical testing. They demonstrated that such an arrangement using fixed band-width narrow-band control could achieve stable closed loop control of an acoustic test system across the entire frequency range of interest from 20 Hz to 10 kHz. They further demonstrated that the system remained stable when using multiple control inputs to control a single output to the transducer array. This multiple-input-single-output control is known as a MISO control system. Application of these techniques to existing DFAT systems led to improved control capability and narrow-band spatial uniformity similar to the ⅓rd octave average uniformity achieved by previous DFAT systems. Even with such improvements, these prior art DFAT systems failed to produce adequate spatial uniformity or low enough coherence to be considered a reliable predictor of results from true reverberant field testing.
In spite of these limitations direct field acoustic testing has seen increasing application due to its flexibility, convenience and the improving reliability of the test results. However, results must still be referenced to reverberant chamber results for absolute confidence. It would therefore be advantageous to provide a DFAT system capable of closely replicating the results of reverberant chamber testing.